1. Field of the Invention
This invention is in the field of hot water fluid recirculation systems, specifically, to an energy saving recirculation system.
2. Description of the Problem Area
In large buildings, such as apartment buildings, hotels/motels, and other commerical buildings, the distance from the domestic hot water supply source (water heater) to the point of use can be quite long. If the water in the piping system has cooled, the user may have to run the water for a long time in order to get hot water. This is inconventient and wastes both water and energy. Water is wasted because the cooled hot water in the pipes usually is discharged to the drain in order to get to hot water. Energy is wasted because the discharged water was heated previously by the hot water supply source.
In order to solve the problems of inconvenience and water waste, hot water recirculating systems are typically installed in larger residential structures and other commercial buildings. By continuously recirculating hot water around a loop, hot water is quickly available at the various points of use throughout the building. An electrically driven pump, typically installed at the end of the return line, draws hot water from the supply side of a hot water supply source and returns it to the cold water inlet of the hot water supply.
While this arrangement allows quick access to hot water and saves water, it is wasteful from an energy standpoint. Even when insulated, the loop and the piping connecting it to faucets and other points of use have a high heat loss. Most loops are uninsulated, and many of these are in unaccessible areas where it is not economically feasible to insulate them. Thus the heat loss from a continuously operating recirculation system can be quite high.
3. Description of the Prior Art
In order to solve the above problems, several systems are in use or have been proposed to turn off the recirculation pump at times. One commonly used method is to install a time clock which operates the recirculation pump during the day and turns the pump off at night. While this system will reduce heat loss at night, it has a severe drawback. This is because recirculation actually is needed most at night. During the day in an apartment building or motel/hotel, people often will use hot water at a number of fixtures throughout the building. This water draw keeps water in the distribution system hot and quickly available everywhere. However at night, very little hot water use usually occurs. If someone does use hot water, and if the recirculating pump is not running, the user may have to wait a long time for the hot water to arrive. In summary, regular hot water use in the day tends to keep water in the distribution system hot so that a recirculation system is needed less, but this is when the time clock runs the recirculation pump. At night, when the recirculation pump would be more useful, the time clock usually turns the pump off.
Another commonly used method is the use of an aquastat, a thermostatic control device for regulating water temperature. The aquastat is installed on the return water line and is set to turn off the pump when it senses that the return water temperature is above a preset point. This system saves very little energy over a system in which the recirculation pump runs continuously. The electrical energy savings are small because the pump motor is typically only a fractional-horsepower motor. The heat energy savings also are small, because while the pump may not operate as much, the distribution loop is still kept fairly hot at all times.
Three additional solutions have been proposed for this problem, however none are in widespread use, nor do any offer a fully practical solution.
Stevenson, in U.S. Pat. No. 4,201,518 (May 6, 1980) describes a manually operated system whereby potential users push a button when hot water is desired. This turns on a recirculation pump and then a timer runs the pump for a preset interval. There are two disadvantages to this system. First, the manual operation requires an extra step; one which is inconvenient and with which users are not familiar. Secondly, the cost of wiring a large buildings, or even small buildings, is prohibitively expensive.
Skaats, in U.S. Pat. No. 4,142,515 (Mar. 6, 1979), describes a system which turns on and off automatically by sensing the pressure in the hot water supply line. However, the system utilizes a pressure regulator on the cold water input line which keeps the line pressure continuously below the lowest level usually supplied by the main cold water supply source. The disadvantages of this system are: (1) the pressure regulator reduces supply system water pressure--this is a severe problem in areas of low water pressure; (2) the cold water pressure regulator is expensive to manufacture and install; (3) the system is not sensitive to low flow rates, e.g., when only one person uses a sink faucet at night; and (4) the system will go through a full cycle and totally heat up the distribution system even if it detects only a brief hot water call.
Lugan, Jr., in U.S. Pat. No. 4,606,325 (Aug. 19, 1986), describes a system primarily designed to save water. This system utilizes a recirculating pump to return unused hot water back to a holding tank rather than allowing it to cool off in the distribution piping. The system is activated by a variety of controls. The disadvantages of this system for the purposes under consideration here are that the system does not automatically sense water usage and that continuous operation of the recirculating pump in some control modes wastes energy.